This invention relates to a ceramic substrate for a packaged electronic device and particularly to via holes through the substrate filled with a highly conductive metal to provide electrical and/or thermal feedthrough to or from the device. Most particularly it relates to a ceramic substrate with filled via holes which provide hermetic sealing to a packaged electronic device.
For some purposes, it is essential that electronic devices employing microcircuitry utilize conducting paths as short as possible to minimize inductance losses in the circuitry. It is also essential that the conducting paths be highly conductive to minimize resistance losses and consequent heat generation.
It is also essential for some purposes, particularly involving relatively high amperage and prolonged continuous use, that heat be transferred away from the electronic devices in a quick and effective manner.
Finally, it is also essential for some purposes, that the aforementioned necessary electrical and/or thermal conductivities be obtained without the sacrifice of hermetic sealing in the package because moisture and other constituents of ambient air can be detrimental to sensitive microcircuitry and because some devices are intended to operate in even more hostile environments.
To deal with these problems, the prior art has gone to via holes in ceramic substrates, filled with a conductive metal.
U.S. Pat. No. 4,942,076, granted Jul. 17, 1990 to Ramachandra M. P. Panicker et al. discloses a method for making metal filled via holes in a ceramic substrate by squeegeeing a tungsten paste containing a binder into the via holes in the substrate, sintering to burn off the paste binder to leave a porous tungsten mass in each via hole, squeegeeing a copper paste on the top of the sintered tungsten in the via holes and reflowing the copper in the paste into the pores of the sintered tungsten. It is disclosed that the amount of binder in the tungsten paste (and thereby the amount of porosity of the sintered tungsten) can be adjusted so that the thermal coefficient of expansion of the tungsten/copper composite matches that of the ceramic substrate to provide stability against separation with temperature changes.
U.S. Pat. No. 4,861,641, granted Aug. 29, 1989 to Brian C. Foster et al. discloses a method for making a fired ceramic substrate with metallized vias by punching via holes in a green tape, filling the punched holes with a tungsten ink and then firing the green tape with its filled via holes. To obtain hermeticity, the co-fired tungsten-filled vias are then further treated by depositing another metal thereon by brush coating and heating or by electroless deposition; and this further treatment is repeated, as necessary, until hermeticity is achieved.
U.S. Pat. No. 4,732,780, granted Mar. 22, 1988 to Stephan P. Mitoff et al. discloses producing a hermetic feedthrough in a ceramic substrate by providing a sheet of liquid phase sinterable ceramic composition having a feedthrough hole, filling the hole with refractory metal metallization material, firing the resulting structure to produce a sintered substrate with adherent metallization comprising refractory metal and glass and then contacting the refractory metal with an electrically conductive intrusion metal and heating the resulting structure so that the intrusion metal melts and displaces the glass.
U.S. Pat. No. 4,131,516, granted Dec. 26, 1978 to Peter Bakos et al. discloses a ceramic substrate with via holes which are first primed with a palladium coating on the inside surfaces of the holes, then coated with an iron film onto the palladium and finally filled with copper flowed into the holes by melting. The iron film is disclosed as essential in order to achieve good adhesion between the copper and the palladium.
U.S. Pat. No. 5,113,315, granted May 12, 1992 to Michael L. Capp, et al. discloses, inter alia, via holes filled with copper for the purpose of thermal conductivity to pass generated heat to heat sink structures. The copper in the via holes, deposited initially by electroless coating and thereafter by electrodeposition is shown in the drawings as completely filling the via holes; and grossly it does fill the holes. However, the filling is not complete because, inherently, complete filling of a hole cannot be obtained by electrodeposition. In electrodeposition, metal can be deposited onto a cathode only as long as the cathode surface is available to the electrolyte solution. Once the hole is almost closed and the flow of electrolyte solution is substantially blocked, electrodeposition cannot continue and the hole remains less than totally filled.
U.S. Pat. No. 5,100,714, granted Mar. 31, 1992 to Kalman F. Zsamboky discloses the metallizing of a ceramic substrate, which may contain via holes, by electroless coating followed by electrodeposition and finally by a heating step to a temperature just below the melting temperature of the electrodeposited metal, specifically at the melting point of the eutectic composition of the metal and its oxide. In the case of electrodeposited copper, which melts at 1085.4.degree. C., the heating step is carried out at a temperature between about 1066.degree. C. and 1075.degree. C. At this temperature, the copper within the via hole does not melt and the hole remaining after electrodeposition is not filled.
Those of the foregoing patents which deal with hermetic sealing achieve their results by complex and expensive procedures and, in some cases, with composite filler materials which have poorer electrical and thermal conductivities than copper, silver, gold or alloys thereof.